5910 UV-ABSORBING ORGANIC CONSTITUENTS*

5910 A. Introduction

1. Applications

Because some organic compounds commonly found in water

and wastewater (e.g., lignin, tannin, humic substances, and various aromatic compounds) strongly absorb ultraviolet (UV) radiation, UV absorption is a useful surrogate measure of such

constituents in fresh waters,1–3 salt waters,4–6 and wastewaters.7,8 Strong correlations may exist between UV absorption and

organic carbon content, color, and precursors of trihalomethanes

(THMs) and other disinfection byproducts.9,10 UV absorption

has been used to monitor industrial wastewater effluents11 and to

evaluate the effectiveness of coagulation,10 carbon adsorption,12–14 and other water treatment processes that remove organics.10,15 Specific absorption (the ratio of UV absorption to

organic carbon concentration) has been used to characterize

natural organic matter.10,16–18 Some regulatory programs include

specific UV absorbance (SUVA)—a ratio of UV absorption to

dissolved organic carbon concentration—to help control disinfection byproducts.

Although UV absorption can be used to detect certain individual organic contaminants after separation [e.g., by high-performance liquid chromatography (HPLC)], as described in Part

6000, the method described here is not suitable for detecting

trace concentrations of individual chemicals. Rather, it provides

an indication of the aggregate concentration of UV-absorbing

organic constituents.

2. References

1. DOBBS, R.A., R.H. WISE & R.B. DEAN. 1972. The use of ultra-violet

absorbance for monitoring the total organic carbon of water and

wastewater. Water Res. 6:1173.

2. WILSON, A.L. 1959. Determination of fulvic acids in water. J. Appl.

Chem. 9:501.

3. COOPER, W.J. & J.C. YOUNG. 1984. Chemical non-specific organic

analysis. In R.A. Minear & L. Keith, eds. Water Analysis, Vol. 3.

Academic Press, New York, N.Y.

4. OGURA, N. & T. HANYA. 1968. Ultra-violet absorbance of the seawater, in relation to organic and inorganic matters. Int. J. Oceanol.

Limnol. 1:91.

5. OGURA, N. & T. HANYA. 1968. Ultra-violet absorbance as an index

of pollution of seawater. J. Water Pollut. Control Fed. 40:464.

6.. FOSTER, P. & A.W. MORRIS. 1971. The use of ultra-violet absorption

measurements for the estimation of organic pollution in inshore sea

waters. Water Res. 5:19.

7. BUNCH, R.L., E.F. BARTH & M.B. ETTINGER. 1961. Organic materials

in secondary effluents. J. Water Pollut. Control Fed. 33:122.

8. MRKVA, M. 1971. Use of ultra-violet spectrophotometry in the

determination of organic impurities in sewage. Wasserwirtsch. Wassertech. 21:280.

9. SINGER, P.C., J.J. BARRY, III, G.M. PALEN & A.E. SCRIVNER. 1981.

Trihalomethane formation in North Carolina drinking waters.

J. Amer. Water Works Assoc. 73:392.

10. EDZWALD, J.K., W.C. BECKER & K.L. WATTIER. 1985. Surrogate

parameters for monitoring organic matter and THM precursors.

J. Amer. Water Works Assoc. 77(4):122.

11. BRAMER, H.C., M.J. WALSH & S.C. CARUSO. 1966. Instrument for

monitoring trace organic compounds in water. Water Sewage Works

113:275.

12. BISHOP, D.F., L.S. MARSHALL, T.P. O’FARRELL, R.B. DEAN,

B. O’CONNOR, R.A. DOBBS, S.H. GRIGGS & R.V. VILLIERS. 1967.

Studies on activated carbon treatment. J. Water Pollut. Control Fed.

39:188.

13. SONTHEIMER, H.N., E. HEILKER, M.R. JEKEL, H. NOLTE &

F.H. VOLIMER. 1978. The Mu ̈lheim process. J. Amer. Water Works

Assoc. 70(7):393.

14. SUMMERS, R.S., L. CUMMINGS, J. DEMARCO, D.J. HARTMAN, D. METZ,

E.W. HOWE, B. MACLEOD & M. SIMPSON. 1992. Standardized Protocol for the Evaluation of GAC, No. 90615. AWWA Research

Foundation & American Water Works Assoc., Denver, Colo.

15. KRASNER, S., P. WESTERHOFF, B. CHEN, B.RITTMANN, S. NAM &

G. AMY. 2009. Impact of wastewater treatment processes on organic

carbon, organic nitrogen and DBP precursors in effluent organic

matter. Environ. Sci. Technol. 43(8):2911.

16. THURMAN, E.M. 1985. Organic Geochemistry of Natural Waters.

Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, Netherlands.

17. OWEN, D.M., G.L. AMY & Z.K. CHOWDHURY. 1993. Characterization

of Natural Organic Matter and Its Relationship to Treatability, No.

90631. AWWA Research Foundation & American Water Works

Assoc., Denver, Colo.

18. WEISHAAR, J.L., G.R. AIKEN, B.A. BERGAMASCHI, M.S. FRAM, R. FUJII

& K. MOPPER. 2003. Evaluation of specific ultraviolet absorbance as

an indicator of the chemical composition and reactivity of dissolved

organic carbon. Environ. Sci. Technol. 37(8):4702.

* Approved by Standard Methods Committee, 2011.

Joint Task Group: Suzanne M. Teague (chair), Ali Haghani, Edward W.D.

Huffman, Jr., Louis A. Kaplan, Ilke E. McAliley, Mark A. Schlautman.

1
5910 B. Ultraviolet Absorption Method

1. General Discussion

a. Principle: UV-absorbing organics in a sample absorb UV

light in proportion to their concentration. Filter samples to control particle-related variations in UV absorption. The sample’s

pH should be adjusted before filtration; however, certain organic

constituents (e.g., humic substances) may be lost to filters, suspended particles, or vessel walls at lower pH values.1 When

filtering samples for SUVA determination, do not adjust pH

before filtration.

UV absorption historically has been measured at 253.7 nm

(often rounded to 254 nm), but the choice of wavelength is

arbitrary. Experienced analysts may choose a wavelength that

minimizes interferences while maximizing absorption by the

compound(s) of interest. If using a wavelength other than 253.7

nm, state that wavelength when reporting results.

b. Interferences: The primary interferences in UV-absorption

measurements are from colloidal particles and UV-absorbing

inorganics—notably iron, nitrate, nitrite, and bromide. Certain

oxidants and reducing agents (e.g., ozone, chlorate, chlorite,

chloramines, and thiosulfate) also absorb ultraviolet light at

253.7 nm. Many natural waters and treated drinking waters have

been shown to be free of these interferences.

Evaluate and correct for UV absorption contributed by specific

interfering substances. If cumulative corrections exceed 10% of

the total absorption, select another wavelength and/or method.

Also, UV absorption by organic matter may vary when pH is

below 4 or above 10, so avoid these ranges.2

A UV absorption scan from 200 to 400 nm can be used to

determine the presence of interferences. Typical absorption

scans of natural organic matter are featureless curves of increasing absorption with decreasing wavelength. Sharp peaks or irregularities in the scan may indicate inorganic interferences.

Because many organic compounds in water and wastewater (e.g.,

carboxylic acids and carbohydrates) do not absorb significantly

in UV wavelengths, correlate UV absorption to dissolved organic carbon (DOC) or soluble chemical oxygen demand (COD).

However, use such correlations with care because they may vary

from water to water, seasonally or during storms on the same

water, and between raw and treated waters. In addition, chemical

oxidation of the organic material (e.g., ozonation, chlorination)

may reduce UV absorption without removing organics and thus

may change correlations. Because UV absorption and correlations with UV absorption are site-specific, they may not be

comparable from one water source to another.

c. Minimum detectable concentration: The minimum detectable concentration cannot be determined rigorously because this

is a nonspecific measurement. Also, the minimum detectable

concentration of a particular constituent depends on the relationship between UV absorption, the desired characteristic (e.g.,

trihalomethane formation potential or DOC), and any interfering

substances. For precise measurement, select cell path length to

provide a minimum absorbance of approximately 0.005 cm1

and a maximum absorbance of approximately 0.950 cm1

. Dilute high-strength samples to fall within the determined range.

d. Sampling and storage: Collect samples in amber glass

bottles that have been washed, thoroughly rinsed with organicfree water, and baked at 400°C for at least 1 hour. Seal with

tetrafluoroethylene (TFE)-lined caps. If individual TFE septa are

used, wash and rinse thoroughly with organic-free water, wrap in

foil and bake at 100°C for 1 hour or soak the septa in 10%

sodium persulfate for 1 hour at 60°C and rinse thoroughly with

organic-free water.3 Pre-cleaned containers from commercial

vendors may also be available. Store samples at 6°C; samples

requiring shipping should be transported on ice. Adjust pH (if

needed) and analyze within 48 hours.

Samples may be filtered in the field. If samples require pH

adjustment, make the adjustment before filtration. Do not adjust

pH of samples for SUVA determination.1

2. Apparatus

a. Spectrophotometer, for use between 200 and 400 nm, with

matched quartz cells providing a light path of 1 cm. For lowabsorbance samples, use a path length of 5 or 10 cm. A scanning

spectrophotometer is useful.

b. Filter: Use a glass fiber filter* without organic binder, or

use TFE, polycarbonate, or silver filters. Polyethersulfone (PES)

or hydrophilic polypropylene (HPP) filters are also recommended.4 Other filters that neither sorb UV-absorbing organics

of interest nor leach interfering substances (e.g., nitrate or organics) into the water may be used, especially if colloidal matter

must be removed. The specific filter used should be reported with

the data. Practical filter diameters are 2.2 to 4.7 cm, although this

may vary. Pre-rinse filter with a sample of organic-free water to

remove soluble impurities. As an option, glass fiber filters may

be combusted at high temperatures (450 to 500°C for at least

2 hours) before rinsing. If alternate separation techniques, filters,

or filter preparations are used, demonstrate that equivalent results

are produced.

Filter pore size will influence test results, especially in raw

waters. When filtering samples for SUVA determination, check

the applicable regulatory constraints to see if a filter with an

absolute pore size of 0.45 m is required, because glass fiber

filters generally do not meet this requirement. For highly turbid

samples, pre-filters (including glass fiber) may be used to remove

interferences and minimize clogging of 0.45-m filters.5

c. Filter assembly, glass, TFE, stainless steel or other material

that neither sorbs UV-absorbing organics nor leaches interfering

substances, capable of holding the selected filters. Polypropylene

syringes with syringe-type filter holders or peristaltic pumps

with in-line filter holders may be used as an alternative to

vacuum filtration.3

3. Reagents

a. Organic-free water: Reagent water containing less than

0.010 cm1 of UV-absorbing compounds. For optimum performance, reagent water of less than 0.0045 cm1 is recommended.

* Whatman grade 934AH; Gelman type A/E; Millipore type AP40; ED Scientific

Specialties grade 161; or other products that give demonstrably equivalent results.

UV-ABSORBING ORGANIC CONSTITUENTS (5910)/Ultraviolet Absorption Method

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b. Hydrochloric acid (optional), HCl, 0.1N.

c. Sodium hydroxide (optional), NaOH, 0.1N.

d. Phosphate buffer (optional): Dissolve 4.08 g dried anhydrous KH2PO4 and 2.84 g dried anhydrous Na2HPO4 in 800 mL

organic-free water. Verify that pH is 7.0 and dilute to 1 L with

organic-free water. Store in a brown glass bottle at 4°C. Prepare

fresh weekly (or more frequently if microbial growth is observed).

e. Organic carbon stock solution: Dissolve 2.1254 g anhydrous primary-standard grade potassium biphthalate, C8H5KO4

[also known as potassium hydrogen phthalate (KHP)], in organic-free water and dilute to 1000 mL (1.00 mL  1.00 mg

carbon). Commercially prepared standards are also available.

4. Procedure

a. Sample volume: Select sample volume based on cell path

length, volume needed for filter rinsing, and/or dilution required

to produce a UV absorbance between 0.005 and 0.950 cm1

. For

most applications, a 50- to 125-mL sample is adequate.

b. Sample preparation: Wash filter and filter assembly by

passing a sufficient volume of organic-free water through the

filter (to waste) to yield a filter blank of 0.010 cm1

. For

specific applications and correlations, sample pH may be adjusted with HCl or NaOH. In poorly buffered samples, an appropriate non-UV absorbing buffer system (e.g., a phosphate

buffer) may be used. Take care to avoid precipitate formation

during pH adjustment. UV absorbance of fulvic acid solutions

apparently remains constant between pH 4 and 10.2 Samples

analyzed for SUVA determination should be run without adjusting the pH of the original sample. Report sample pH value used

with recorded absorbance. Once sample pH has been adjusted

and/or measured, rinse filter with sample and discard filtrate. If

using 4.7-cm PES or HPP filters, they can be adequately rinsed

with 25 mL sample to bind sites that may adsorb UV-absorbing

constituents.4 This volume is also adequate for glass fiber filters.

Smaller filters may require proportionately less. If other filter

materials are used, confirm whether these volumes are adequate

or if more is needed. Analyze the subsequent filtrate as a sample.

Prepare an organic-free water filter blank and the sample in an

identical manner.

c. Spectrophotometric measurement: Let spectrophotometer

equilibrate according to manufacturer’s instructions. Set wavelength to 253.7 nm (or 254 nm) and adjust spectrophotometer to

read zero absorbance with organic-free water. Measure UV

absorbance of at least two filtered portions of sample at room

temperature.

5. Calculation

Report mean UV absorption in units of cm1 using the following notation. To report units in m1

, multiply the equation by

100.

UVλ

pH 

A

b

D

where:

UV

pH  mean UV absorption, cm-1 (subscript denotes wavelength

used, nm, and superscript denotes pH used if other than

7.0),

b  cell path length, cm,

A  mean absorbance measured, and

D  dilution factor resulting from pH adjustment and/or dilution with organic-free water.

D final sample volume

initial sample volume

Correct results for absorption contributed by known interfering substances. If UV absorption contributed by interfering substances exceeds 10% of total UV absorption, do not use UV

absorption at 253.7 nm as an indicator of organics.

6. Quality Control

The quality control (QC) practices considered to be an integral

part of each method are summarized in Table 5020:I. Additionally, follow all QC described in the method itself.

a. Replicate measurements: Use at least two portions of filtered sample.

b. Duplicate analyses: Analyze every tenth sample in duplicate (i.e., duplicating the entire procedure) to assess method

precision. Calculate relative percent difference (RPD) for duplicate analyses (D1 and D2, where each duplicate is the average of

replicate readings) using the formula below:

%RPD  D1  D2 

D1  D2/2

 100

For UV values 0.045 cm1

, the RPD limit is 10%. For UV

values 0.045cm1

, the RPD limit is 20%.

c. Baseline absorbance: Check system baseline UV absorbance after every 10 samples by measuring the absorbance of an

organic-free water blank. A non-zero absorbance reading for the

blank may indicate the need for cell cleaning, problems with the

reference cell if a dual-beam instrument is being used, or a

variation in spectrophotometer response caused by heating or

power fluctuations over time.

d. Filter blanks: With each batch of filtered samples, analyze

a portion of organic-free water that has been passed through a

rinsed filter. The volume of organic-free water should be equal to

the volume of a sample. Filter blanks should read 0.010 cm1

;

if not, larger rinsing volumes may be required. Also, the appropriateness of the filter and/or the filtering apparatus may need to

be evaluated.

e. Filtrate turbidity check (optional): If a UV value or calculated SUVA value seems unusually high for a specific sample

source, check filtrate turbidity. Turbidities greater than 0.5 nephelometric turbidity units (NTU) indicate that the sample may

require pretreatment steps, such as centrifugation.5

f. Spectrophotometer check: Difficulties in comparing UV

absorption data from different spectrophotometers have been

reported. KHP standards were prepared in pH 7, phosphatebuffered (3d) reagent water without acidification and analyzed in

UV-ABSORBING ORGANIC CONSTITUENTS (5910)/Ultraviolet Absorption Method

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five laboratories. Results suggest acceptable precision (Table

5910:I). These data also are useful for checking spectrophotometer results with KHP standards commonly used for total organic

carbon (TOC) and/or COD analysis. A correlation equation for

this 40-sample data set is:

UV254  0.0144 KHP  0.0018

with correlation coefficient (r

2

)  0.987, UV254 expressed in

cm1

, and KHP expressed as mg/L as C.

This equation can help verify spectrophotometer performance.

For example, if a set of UV254 analyses is performed and results

are in the 0.010 range, prepare a KHP standard of 0.5 mg/L as C.

The projected UV254 of this KHP standard would be 0.009 cm1

.

If measured UV254 is outside the 13% relative standard deviation

(RSD) of 0.009 cm1 (using the RSD in Table 5910:I as a

guide), the spectrophotometer may be suspect and require maintenance. The correlation between UV254 and KHP standards is

presented solely as a useful means of verifying spectrophotometer performance. Prepare at least one KHP standard in the range

of interest and measure it with each analytical run. Commercially

prepared standards may also be available for this purpose.

7. Precision and Bias

Table 5910:I shows interlaboratory precision data for 40 KHP

samples. The RSD ranged from 9.38 to 12.8%.

Single-operator precision data are presented in Table 5910:II

for fulvic acid solutions.6 The RSD ranged from 0.9 to 6%.

Because UV absorption is an aggregate measure of organic

carbon, true standards are unavailable and bias cannot be determined.

The precision of analyses by this method was determined

under the Information Collection Rule (ICR).7 Precision was

determined as RPD for duplicate analyses and was calculated

only when both analyses in the duplicate pair showed concentrations at, or greater than, the Information Collection Rule

minimum reporting level (MRL 0.009 cm1

). Results were as

follows:8

Data Quality

Variable

Percentile

N 10 25 50 (median) 75 90

Precision (RPD) 33306* 0.0 0.0 0.0 2.2 4.9

* 2744 excluded - both samples less than MRL.

NOTE: Median sample result was 0.040 cm-1.

8. References

1. KAPLAN, L.A. 1992. Comparison of high-temperature and persulfate

oxidation methods for the determination of dissolved organic carbon

in freshwaters. Limnol. Oceanogr. 37:1119.

2. EDZWALD, J.K., W.C. BECKER & K.L. WATTIER. 1985. Surrogate

parameters for monitoring organic matter and THM precursors.

J. Amer. Water Works Assoc. 77(4):122.

TABLE 5910:I. PRECISION OF UV ANALYSES AND CORRELATION TO KHP SAMPLES

UV254 Result for Given KHP Sample Concentration*

cm-1

Analysis 0.54 0.93 1.79 4.87 9.61 25.0 50.0 100.0

Laboratory 1 0.008 0.015 0.034 0.079 0.158 0.323 0.638 1.282

Laboratory 2 0.009 0.016 0.026 0.070 0.134 0.401 0.803 1.612

Laboratory 3 0.010 0.017 0.027 0.081 0.161 0.353 0.695 1.343

Laboratory 4 0.007 0.020 0.033 0.070 0.132 0.319 0.750 1.590

Laboratory 5 0.009 0.018 0.030 0.087 0.140 0.394 0.643 1.447

Mean 0.0086 0.0172 0.0300 0.0774 0.1450 0.3580 0.7058 1.4548

Standard deviation 0.0011 0.0019 0.0035 0.0074 .0136 0.0384 .0708 0.1461

Relative standard deviation, %† 12.8 11.1 11.7 9.56 9.38 10.7 10.0 10.0

* KHP sample concentration mg/L as C, measured as in Section 5310C.

† The percent relative standard deviation is given by:

% RSD 

standard deviation (S)

mean (X)  100

TABLE 5910:II. SINGLE-OPERATOR PRECISION FOR UV ABSORPTION

MEASUREMENTS OF FULVIC ACID SOLUTIONS

Result

cm-1

DOC Concentration

mg/L

Replicate No. 2.5 4.9 10.0

1 0.110 0.240 0.480

2 0.120 0.230 0.480

3 0.110 0.240 0.470

4 0.100 0.230 0.480

5 0.110 0.240 0.480

6 0.100 0.240 0.470

7 0.110 0.240 0.480

8 0.110 0.230 0.480

9 0.120 0.240 0.480

10 0.110 0.240 0.480

Mean 0.110 0.237 0.478

Standard deviation 0.0067 0.00483 0.00422

% Relative standard

deviation

6.06 2.05 0.882

UV-ABSORBING ORGANIC CONSTITUENTS (5910)/Ultraviolet Absorption Method

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3. KAPLAN, L.A. 1994. A field and laboratory procedure to collect,

process, and preserve freshwater samples for dissolved organic carbon analysis. Limnol. Oceanogr. 39(6):1470.

4. KARANFIL, T., I. ERDOGAN & M. SCHLAUTMAN. 2003. Selecting filter

membranes for measuring DOC and UV254. J. Amer. Water Works

Assoc. 95(3):86.

5. KARANFIL, T., I. ERDOGAN & M. SCHLAUTMAN. 2005. The impact of

filtrate turbidity on UV254 determination. J. Amer. Water Works

Assoc. 97(5):125.

6. MALLEY, J.P., JR. 1988. A Fundamental Study of Dissolved Air

Flotation for Treatment of Low Turbidity Waters Containing Natural

Organic Matter. Ph.D. thesis (unpublished), Univ. Massachusetts,

Amherst.Line Height Adjustment

7. U.S. Environmental PROTECTION AGENCY. 1996. Monitoring Requirements for Public Drinking Water Supplies: Cryptosporidium, Giardia,

Viruses, Disinfection Byproducts, Water Treatment Plant Data and

Other Information Requirements. Final Rule. Fed. Reg. 61(94):24354.

8. FAIR, P.S., R.K. SORRELL, M. KARAPONDO, B.J. BOHL, C.P. FREBIS &

M.A. FEIGE. 2002. Quality of ICR monitoring data. In M.J. McGuire,

J. McLain & A. Oblensky, eds. Information Collection Rule Data

Analysis. American Water Works Assoc. Research Foundation, Denver, Colo.

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